1. Field of the Invention
The present invention pertains to the formation of a hermetic sealing joint along the target and backing interface by the provision of melted solder around the interfacial area.
2. Prior Art
A customer driven need has been identified to reduce the impact of virtual leaks from solder bonded assemblies on vacuum quality in their sputtering system. The goal is to cut the bond line off from the vacuum by a coherent, fully dense metal seal.
Solder bonding is generally done using either a tin eutectic alloy (such as tin 3.5 weight percent silver) or pure indium. The bonding process is performed in air or under a protective atmosphere, and a solder flux may be used to break oxide barriers on the metal being bonded. Bonding in air or under a protective atmosphere can lead to trapped pockets containing gas. It is possible for porosity within the bond layer to allow the trapped gas to escape over at a low leak rate from the bond line between the target and the backing plate.
The most common approach to eliminate virtual leakage from a solder bond is to eliminate the solder bond entirely. This can be done several ways. The most simple is to make the target/backing plate a monolithic assembly, i.e., one piece. The second method is to diffusion bond the target to the backing plate. Diffusion bonding is performed under high pressure and temperature, and a vacuum exists inside the diffusion bonding assembly prior to the application of heat and pressure. A third method involves electron beam welding of the target to the backing plate. Electron beam welding works if the joint between the target and backing plate is ductile. It is not possible to directly weld aluminum to copper.
It is not practical to solder bond in a vacuum. Thus, trapped pockets of gas are inevitable in a solder bond.
A monolithic assembly is possible when the economic value of the material is not too high, and the target material is sufficiently strong and ductile to bear the structural demands of the backing plate. Deflection can be a problem if the target alloy is not very strong. Some materials are also brittle. Some backing plate designs involve internal cooling channels, and this is the case with one current CVC commercial design. A monolithic assembly would involve redesigning the target/backing plate to a two-piece setup with channels cut in the back of the monolithic piece (similar to the Ulvac ZX-1000).
Diffusion bonding is commonly conducted at temperatures between 300xc2x0 and 6000xc2x0 C. This can be detrimental to the microstructure of some materials and also will leave residual stress in the assembly if the thermal expansion coefficients of the target and backing plate are different. Brittle target materials are difficult to diffusion bond because of the bending moment and stress caused by the differential thermal expansion and the application of pressure at high temperature. Not all materials can be diffusion bonded to copper, aluminum or molybdenum, the most common backing plate materials. The same issue regarding internal cooling channels exists for diffusion bonded targets.
Electron beam welding the target to the backing plate is possible only if the backing plate and target materials will form a ductile alloy.
In accordance with the invention, a solder material such as pure tin or indium is used to hermetically seal the circumferential joint or interface formed between a cylindrical target and a generally cylindrical backing plate.
The invention can be summarized by the following steps:
1) Solder Bond target to backing plate using conventional bonding practice;
2) Clean excess solder from the backing plate/target assembly;
3) Optionally, place a shim of the sealing material (In or Sn) at the target/backing plate interface;
4) Use an Electron Bearn welder (low power in vacuum) and run a molten bead around the target/backing plate interface until at least one complete revolution is made. This target will be on a rotating platter of some kind.
5) A laser may be substituted for the electron beam. A focused energy beam may be used to excite the solder bond without disturbing the solder backing plate or moving the target structure away from the controlled spot. IDX biggest concern for a laser system would be a lack of vacuum to avoid trapped gas in the weld region.
The indium metal (or tin) will wet the surface of the backing plate and the target, and the molten pool will solidify 100% dense because of the speed and vacuum. The welded bead will seal off the bond line and permanently close off the trapped gas areas between the target and backing plate.
Indium may be preferable to tin because of its tendency to wet most materials. Indium wetting may be enhanced by rubbing pure indium against the area to be coated.
This method is different from electron beam welding the target to the backing plate because the target material and the backing plate material do not alloy together. There maybe a very small region at the indium (or tin) metal interfaces form intermetallics which are no different than if the indium or tin had been soldered to those surfaces in the traditional manner. In addition, the target may be removed by simply heating the assembly above the melting point of the solder, and the backing plate can be used again.
Electron beam welding is suggested here because a vacuum is required to operate the welder. Making the hermetic seal in a vacuum environment will reduce the chances of porosity impacting the quality of the seal. Trapped gas may cause an eruption which could compromise the seal.
TIG welding may also work to make this seal. However, there is a higher probability of trapped gas causing bubbles in the seal with TIG welding.
The invention provides advantage since the same technology can be used on other backing plate assemblies besides the current customer design, and because more materials can be bonded together with no virtual leak. The grain structure of the target can also be preserved, and the basic backing plate construction does not have to be modified to achieve the desired result. Backing plates with internal cooling channels can also be used.
It is thought that the following features are new:
1) Modifying the backing plate/target dimensions to promote a hermetic seal with an electron beam welder;
2) Ignoring the content of the majority of the bond area and focusing only on the edge; and
3) Use of an electron beam welder in vacuum to run a solder bead for bonding targets and backing plates.
In one embodiment, an internal reservoir is made by machining a trough in the backing plate near the outer edge of the target. After bonding, the e-beam will be is used to heat the outer edge of the target zone until the solder starts to melt. The trough allows a solder to flow from both sides to fill the zone. While this may cause some undercutting at the very edge of the target, there will be no opening into trapped pockets under the target.
The trough may, for instance be in the form of a groove with an outer diameter of 11.80xe2x80x3 and about a 5xc2x0 taper to a depth of a 0.020xe2x80x3. The tolerance should not be too tight on the taper or the depth as it will be to measure or repeat uses. Between uses, the trough can be prior to the next bonding attempt. It has been found that there is more advantage when the size of the trough is about 2xc3x97 to 10xc3x97 thicker than the average bond line thickness. It is possible that the groove positioned at the outer diameter of the target acts as a sink for the solder and might by itself improve the solidification process to reduce the tendency for edge voids to extend into the bond line and subsequently act as virtual leaks.
By way of example, a commercially available CVC style backing plate design will be used for a soldering experiment. The vacuum quality of the result cannot be checked,
The experiment will be done with a 100% dense target in order to prevent distortion during the bonding process. A 99.99% pure copper target fits this requirement.
It may be worth considering that the copper backing plate is coated with nickel or nickel-vanadium (non-magnetic) to prevent significant alloying of the solder with the backing plate. This would be a phase II modification after we investigate the closure of the gap by the e-beam for vacuum tightness.
A backing plate (31950) was bonded to a 6N copper target, (lot 8V3669-102) using indium solder. The backing plate was modified according to the plan described above.
After bonding, the target was scanned for voids.
After scanning, the target was placed in a fixture in the electron beam welder to allow the target to be spun around its axis so that the electron beam could be scanned over the target/backing plate interface.
Photographic image results suggest that there is more impact from the shape of the groove than from the welding process. However, this was the first attempt at groove design and the welding process.
The process of bonding followed by electron beam melting of the solder at the target/backing plate interface is feasible and no adverse impact is observed.
It is possible that the groove positioned at the outer diameter of the target acts as a sink for the solder and might by itself improve the solidification process to reduce the tendency for edge voids to extend into the bond line and subsequently act as virtual leaks.
When copper backing plates are employed, it may be beneficial to coat the copper backing plate with nickel or nickel-vanadium (non-magnetic) to prevent significant alloying of the solder with the backing plate.
The invention will be further disclosed in the following detailed description which is to be read in conjunction with the appended drawings.